4.6 Downlink transmit diversity

25.2243GPPPhysical layer procedures (TDD)TS

Downlink transmit diversity for PDSCH, DPCH, P-CCPCH, S-CCPCH, PICH, MICH, HS-SCCH, HS-PDSCH, E-AGCH, E-HICH and SCH is optional in UTRAN. Its support is mandatory at the UE.

4.6.1 Transmit diversity for PDSCH, DPCH, HS-SCCH, HS-PDSCH and E-AGCH

The transmitter structure to support transmit diversity for PDSCH, DPCH, HS-SCCH, HS-PDSCH and E-AGCH transmission is shown in figure 1. Channel coding, interleaving and spreading are done as in non-diversity mode. The spread complex valued signal is fed to both TX antenna branches, and weighted with antenna specific weight factors w₁ and w₂. The weight factors are complex valued signals (i.e., w_i = a_i + jb_i ), in general. These weight factors are calculated on a per slot and per user basis.

The weight factors are determined by the UTRAN. Examples of transmit diversity schemes are given in annex B.

Figure 1: Downlink transmitter structure to support Transmit Diversity
for PDSCH, DPCH, HS-SCCH, HS-PDSCH and E-AGCH transmission (UTRAN Access Point)

4.6.2 Transmit diversity for SCH and S-CCPCH

Time Switched Transmit Diversity (TSTD) can be employed as a transmit diversity scheme for the synchronisation channel and/or S-CCPCH.

4.6.2.1 SCH transmission scheme

The transmitter structure to support transmit diversity for SCH transmission is shown in figure 2. P-SCH and S-SCH are transmitted from antenna 1 and antenna 2 alternatively. An example for the antenna switching pattern is shown in figure 3.

Figure 2: Downlink transmitter structure to support Transmit Diversity
for SCH transmission (UTRAN Access Point)

Figure 3: Antenna Switching Pattern (Case 2)

4.6.2.2 S-CCPCH transmission scheme

The transmitter structure to support TSTD for S-CCPCH is shown in figure 3a. The antenna switching pattern is under the control of the Node-B and is not explicitly known to the UE. Switching may only be performed during the guard periods between timeslots.

Figure 3a: Downlink transmitter structure to support TSTD
for S-CCPCH transmission (UTRAN Access Point)

4.6.3 Transmit diversity for beacon channels

When beacon channels use burst type 4, SCTD shall not be applied.

When beacon channels use burst type 1, Space Code Transmit Diversity (SCTD) for beacon channels may be employed optionally in the UTRAN. The support is mandatory in the UE except for the case where the UE only supports burst type 4. The use of SCTD will be indicated by higher layers. If SCTD is active within a cell:

– SCTD shall be applied to any beacon channel, and

– the maximum number K_Cell of midambles for burst type 1 that are supported in this cell may be 8 or 16, see [8]. The case of K_Cell = 4 midambles is not allowed for this burst type.

4.6.3.1 SCTD transmission scheme

The open loop downlink transmit diversity scheme for beacon channels is shown in figure 4. Channel coding, rate matching, interleaving and bit-to-symbol mapping are performed as in the non-diversity mode. In Space Code Transmit Diversity mode the data sequence is spread with the channelisation codes and and scrambled with the cell specific scrambling code. The spread sequence on code is then transmitted on the diversity antenna. The power applied to each antenna shall be equal.

Figure 4: Block Diagram of the transmitter SCTD